SUMMARY OF WORK In collaboration with Eduardo Rios, we completed initial work on estimation of the time course of calcium release in skeletal and cardiac muscle calcium sparks, giving clear evidence of the decay of release flux during the spark. Experimental studies indicated that, skeletal sparks may have varying morphologies indicative of spatially extended release that may be either synchronous or propagating depending on conditions. Measurements of sarcoplasmic reticulum calcium showed that lumenal depletion is probably modest in cardiac sparks and quite small in skeletal sparks. In additions, we have begun turning our attention to the local calcium release events which are involved in rate control in pacemaker cells, and which are larger than sparks and smaller than waves. We have developed a method to study these events in steady-state in skinned cells, and have begun characterizing their statistics with an eye to understanding the source of their periodicity. All of these developments have led to the conclusion that the basic Monte Carlo model needs to be extended to take into account the more complex distribution of RyR1 and Ryr3 at the skeletal triad junction, as weel as the propagation of activation among RyR clusters at the multi-sarcomere level. This is a major computational undertaking, made possible by the large advance in computer speed over the past several years. It involves an extensive revision of the simulation code, which is underway at the present time. We have also opened a collaboration with Dr. Rafael Rosales of University of Sao Paulo, to adapt his hidden-Markov method to the identification of the gating scheme underlying our L-type calcium channel patch-clamp measurements.